1. Field of the Invention
The present invention relates to an image forming method which is applied to an electrophotographic method, an electrostatic recording method and an electrostatic printing method. Specifically, the invention relates to an image-forming method for obtaining an image from a digital electrostatic latent image.
2. Description of Related Art
In the electrophotographic method, a toner in a developer is adhered to an electrostatic latent image formed on a latent image support (hereinafter sometimes referred to as a "photoreceptor"), transferred onto a paper or a plastic film as a transfer material, and fixed through heating to form an image.
Coloration has been progressed in a printer or a copier using an electrophotographic method. Further, a latent image is rendered fine to improve resolution of the apparatus. Accordingly, in a full-color copier in which a digital latent image is developed, transferred and fixed using a color toner, a toner having a small particle diameter of from 7 to 8 .mu.m is employed to achieve an image of a high quality to some extent. However, the further improvement of a fine line reproducibility or a gradation has been required by more reducing the particle diameter of the toner.
Meanwhile, as a photoreceptor used in the electrophotographic method, an inorganic photoreceptor has been so far used. However, in recent years, the studies and the development of an organic photoreceptor (OPC) which has an organic photoconductive layer on the surface and which is less costly and excellent in a productivity and a disposal. Among others, a so-called functionally separate laminated photoreceptor obtained by laminating a charge generation layer and a charge transfer layer has been put to practical use.
It is deemed that the life of the organic photoreceptor ends mainly when the image defect owing to the staining of the surface and the image defect owing to the wearing-out of the surface layer occur. Therefore, an organic photoreceptor of which the surface is less stained and less worn out has been in demand for prolonging the life thereof.
It is indeed unavoidable that the surface of the organic photoreceptor is stained with a toner and an external additive to some extent. Ordinarily, the staining is prevented by appropriately wearing out the surface of the organic photoreceptor with an external additive. As the hardness of a toner or an external additive is increased and the particle diameter thereof is increased, the wearing-out of the surface of the organic photoreceptor tends to be increased. Accordingly, in order to prevent the surface of the organic photoreceptor from being stained, an external additive having an appropriate hardness and an appropriate size is generally used.
At this time, when an amount of a toner consumed is increased, an amount of a toner that is passed in contact with the organic photoreceptor is increased, and an amount of an external agent fed to the organic photoreceptor is also increased naturally to accelerate the staining and the wearing-out of the surface of the organic photoreceptor. Further, when the particle diameter of the toner is decreased, the amount of the external additive is sometimes increased for improving a fluidity, accelerating the staining and the wearing-out of the external additive.
Consequently, it is required that the amount of the toner consumed and the amount of the external additive are decreased to prevent the properties of the photoreceptor from being worsened.
The organic photoreceptor has the non-uniformity of the surface to some extent for reasons of the production, and the electrostatic latent image formed on the surface is thereby influenced, with the result that the unclear image is naturally formed. This unclear electrostatic latent image is a defect in the digital electrophotographic method.
In a development nip portion in which the development is conducted, a toner is flown and reversely flown repetitively between an organic photoreceptor and a developer support by an action of a development electric field. When the development electric field is not activated immediately after passage through the development nip portion, the image structure of the electrophotographic latent image formed on the surface of the organic photoreceptor is determined. When the electrostatic latent image is unclear, the sharpness is worsened immediately after passage through the development nip portion to cause the disorder of the image. Especially, it is considered that a toner having a large particle diameter has a relatively low non-electrostatic adhesion and tends to entrain the action of the development electric field, decreasing the sharpness and causing the disorder of the image. Meanwhile, when the particle diameter is decreased, the non-electrostatic adhesion is increased, and it becomes hard to fly the toner from the carrier to the photoreceptor.
On the other hand, in a transferring step of transferring the toner image developed onto a transfer material, the toner is flown from the organic photoreceptor to the transfer material by the action of the transfer electric field in the transfer nip portion. However, a toner having a large amount of charge tends to be scattered, causing the disorder of the image. It is considered that since a toner having a large particle diameter has a relatively low adhesion, it also tends to entrain the action of the transfer electric field, causing the disorder of the image. Meanwhile, when the particle diameter of the toner is small, it becomes hard to transfer the image from the photoreceptor, decreasing the transferring property.
Consequently, in the image-forming method using the organic photoreceptor, a toner of a small particle diameter which can appropriately control the non-electrostatic adhesion and the charge amount of the toner and which does not cause the disorder of the image has been in demand.